Those contra-angle heads intended for use by a dentist to drive interchangeably one or more of drills, burs, broaches, files and other tools used in dentistry incorporate rugged and usually precise mechanisms for accepting and locking the tools in place, driving them at desired speeds, and releasing them for removal quickly and reliably, but for the most part such contra-angle heads are bulky and large, have awkward shapes, and are heavy. The positions into which a dentist can put a tool in the mouth of a patient are restricted, and often a tool can be used to treat a patient only at the expense of great discomfort to the patient, and risk of harming surrounding tissue that is not directly involved in the procedure being undertaken.
Projecting parts, such as extending gear boxes and external latching levers, which are commonly found on contra-angle heads currently in use tend to aggravate such discomfort and risks. For example, in U.S. Pat. No. 3,369,298 a clutch is permanently rotatably fixed in a stub-housing oriented transversely to the principal housing, and gears to rotate the clutch on an axis transversely to the main drive shaft axis are housed in the stub housing, which is enlarged for that purpose. A tool can be removably inserted into the clutch from one end of the stub housing, and a lock lever is provided, externally, at the other end.
Attempts to provide smaller and smoother contra-angle heads with interchangeable tools have lead, in one instance, to a construction in which the tool is inserted through the stub housing from the locking end, and a threaded cap is provided at the same end to perform the locking function. As it is attempted to make a contra-angle head smaller, these parts, especially removable caps, become so small that they are easily lost, wasting the time of the dentist, and so expensive that their prices are difficult to justify in times of rising prices of other more important medical procedures.